1. Field of Invention
This invention is related to the field of electrical rotating machinery for the conversion between electrical energy and mechanical energy.
2. Prior Art
The basic principal of nearly all electrical rotating machinery is that a current of electrical charge located within a magnetic field will experience a force perpendicular both to the flow of charge and the lines of force of the magnetic field. Most electrical rotating machines make use of this principal by generating a magnetic field directed radially about a cylinder, causing current to flow axially along said cylinder, thus developing a tangential force which causes said cylinder to turn. Other geometries are possible, for example, so called `axial flux` machines make use of a magnetic field generally parallel to the axis of rotation, and a generally radial current flow, again causing tangential force and thus causing rotation. If a conductor is forced through a magnetic field by some sort of external prime mover, then an electrical current can be caused to flow; this is the principal of the generator.
In the method of the AC induction motor, a rotating magnetic field is produced by the stator or stationary portion of the machine. This rotating magnetic field has two functions. First, it interacts with current carried by conductors of the rotor, causing the rotor to turn. Second, it produces said rotor currents by means of transformer action. Thus the rotor needs no connections to means of electrical supply, and is simply supported by bearings which allow free rotation. Such design simplifies motor construction, and greatly enhances motor reliability. The essence of the AC induction motor, and by extension the AC induction generator, is the production of a smoothly rotating magnetic field in the stator.
The rotating magnetic field is produced by coils of wire or windings, suitably placed in the stator. Each winding, when energized with a direct current, would produce a fixed magnetic field. By energizing a winding with a sinusoidal alternating current, a smoothly varying magnetic field of fixed orientation may be produced. Finally, by placing several windings of differing orientation within the same stator, and energizing said windings with alternating currents of differing phase, a rotating magnetic field may be produced which is the sum of the time varying fixed orientation magnetic fields generated by each winding/phase.
The difficulty with this approach is that production of a smoothly rotating magnetic field depends upon two factors. First, the fixed magnetic field generated by each winding must have a generally sinusoidal distribution of intensity. Second, the alternating currents used to energize the winding must also be sinusoidal in nature. Any deviation from the ideal sinusoidal relations will produce harmonic rotating fields, that is magnetic fields which rotate at a different rate and/or direction from the fundamental field. These rotating fields are superimposed and added to the fundamental rotating magnetic field. Each of these harmonic fields exerts its own pull upon the stator, reducing power output, and each results in its own electrical losses, again making the motor less efficient.
Harmonic fields generated by the non-sinusoidal nature of the field generated by each winding are termed spatial harmonics or air-gap harmonics. Harmonic fields generated by non-sinusoidal drive wave-forms are termed temporal harmonics.
Methods for the analysis of harmonic rotating fields in three phase induction machines are well known, and may be found in many textbooks on rotating machinery.
Spatial harmonics are mitigated in three phase machines through the use of distributed windings and chorded windings. These are winding techniques which result in a decrease in the fundamental efficiency of the machine, increasing resistance losses in the windings by up to fifteen percent or more. However these winding techniques disproportionately reduce the strength of harmonic fields. The net result is that both machine operation and total efficiency is improved.
Temporal harmonics are only considered a problem with the advent of inverter based variable frequency motor control systems. These systems produce wave-forms rich in harmonic content. Mitigation of these harmonics has been limited to improving the characteristics of the inverter systems, reducing the harmonic content of the output wave-forms through pulse shaping and higher switching frequencies.
Temporal harmonics also become a problem when high magnetic saturation levels are used. Ferromagnetic materials are used in motor construction because of the much higher magnetic fields which are developed for a given current flow. However, as the magnetic field strength is increased, the relationship between current flow and generated magnetic field becomes non-linear. Even if a perfectly sinusoidal alternating current is applied to a winding, temporal harmonics in the resulting magnetic field will be generated. The intensity of these harmonics increases with increasing saturation, thus setting a limit on the saturation levels which may be used. Winding techniques cannot effectively reduce the strength of harmonic fields generated by high saturation in three phase machines.
The closest known prior art is ben-Aaron, "Polyphase Induction Motor System and Operating Method", U.S. Pat. No. 4,749,933, Date of Patent Jun.7, 1988. Ben-Aaron devised a method of pole changing wherein the number of magnetic poles of the stator magnetic field might be dynamically varied. Ben-Aaron made use of a plurality of pulse width modulated sine wave inverters to provide alternating current to individual stator inductors. Each inductor extendes the length of a stator slot, at which point it is connected to a conductive ring representing the star point of the polyphase circuit. There are two major deficiencies to the approach disclosed in ben-Aaron(1988). The first is that his induction motor is a low voltage, high current device, requiring expensive switching devices which are underutilized owing to the low voltage used. The second flaw is that pole changing does not enhance the torque capabilities of flux density limited machines. The present inventor has determined experimentally that lower pole counts are beneficial to efficiency at high speeds and beneficial to torque at low speeds.
Mention of polyphase induction machines is also common in the art, with the general understanding that polyphase means three, or possibly two, phases. The EASA formulae for winding calculations, for example, do not restrict the number of phases in any way. However, textbook analysis of harmonic interactions are performed only on the three phase machine. Electronic drive systems almost invariably are three phase systems, or perhaps rarely single phase systems. Induction machine drive systems of any desired phase count are totally consistent with basic motor physics and design theory, however current motor design practice does not make use of high phase order systems, and there are heretofore unrecognised benefits of high phase order systems.